The optical properties of nine chemically analyzed samples of natural plagioclases have been accurately determined before and after changing them to high-temperature modifications by heat treatment. The change in Nx accompanying the structural change is slight in the composition range An 0 to An 20 and negligible from An 20 to An 100. Ny and Nz change measurably in the composition range An 0 to An 20, but negligibly in the remainder of the composition range. Measurements of principal refractive indices can therefore give a reliable estimate of the composition of a plagioclase regardless of its structural state.

The changes in optic axial angle accompanying the structural changes are such that, given composition, measurements of optic axial angle serve to distinguish low-temperature and high-temperature plagioclases in the composition ranges An 0 to about An 40 and An 60 to about An 90.

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